Nuclear fusion is the process by which nuclear reactions between light elements form heavier ones (up to iron). Substantial amounts of energy are released in cases where the interacting nuclei belong to elements with low atomic numbers.

In 1939 the physicist Hans A. Bethe suggested that much of the energy output of the Sun and other stars results from energy-releasing fusion reactions in which four hydrogen nuclei unite and form one helium nucleus. During the early 1950s American researchers produced the hydrogen bomb by inducing fusion reactions in a mixture of the heavy hydrogen isotopes deuterium and tritium, the reactions being ignited by the extremely high temperatures created in the fission reaction of an atomic bomb. More recently, scientists have sought to devise a practical method of controlled nuclear fusion, in which deuterium and tritium nuclei would combine to form helium nuclei under controlled high temperatures rather than in the uncontrollable heat of a detonating atomic bomb. Controlled nuclear fusion would provide a relatively inexpensive alternative energy source for electric-power generation and thereby help conserve the world's dwindling supply of oil, natural gas, and coal. Fusion also would be more advantageous than nuclear fission (q.v.), another kind of energy-producing nuclear reaction that occurs when a heavy nucleus such as the isotope uranium-235 absorbs a neutron, becomes unstable, and splits into two lighter nuclei. Deuterium, the primary fuel for a fusion-power system, is far more abundant and cheaper than any of the materials required for fission reactions, since it can be extracted from ordinary water. (Eight gallons of water contain about one gram of deuterium, which has an energy content equivalent to roughly 9,500 litres [2,500 gallons] of gasoline.) Many experts, however, believe that controlled fusion will not be achieved for some years because of various technical difficulties.

A fusion reaction can occur only if two nuclei approach each other within a distance on the order of 10^{-13 centimetre. At such a short distance it is possible for the nuclear forces of attraction to overcome the electrostatic forces of repulsion that result from the presence of positive electric charges on both nuclei. Because the forces of repulsion are so effective in keeping nuclei apart, reactions useful for controlled fusion seem largely limited to deuterium and tritium, nuclei with the lowest possible charge. Controlled energy-releasing reactions can be produced by heating a plasma (gas consisting of unbound electrons and an equal number of positively charged nuclei) of a deuterium-tritium mixture to many millions of degrees kelvin. Such high temperatures will not only induce but also sustain thermonuclear reactions so as to produce enough energy for electric-power generation. However, it has proved extremely difficult to contain plasmas at the high temperature levels required to achieve self-sustaining fusion reactions because the hot gases tend to expand and escape from the enclosing structure. Large-scale fusion reactor experiments have been conducted in various countries, including the United States and Russia, in an effort to develop a means of overcoming this problem.}

Excerpt from the Encyclopedia Britannica without permission.